In general, such low speed and high torque geared motors are employed in a wide variety of industrial robots. A representative known geared motor is described in Japanese Laying-open Publication No. 57-208838. The known geared motor comprises first and second arms connected to each other and pivotable with respect to each other about an axis of rotation, and a housing accommodating a geared motor therein. The geared motor comprises a pair of first spaced and parallel ring gears secured to the housing, a pair of spaced and parallel planetary gears held in meshing engagement to the first ring gears, respectively, a pair of second spaced and parallel ring gears positioned axially outwardly of the first ring gears and having axes of rotation substantially in axial alignment with the axes of the first ring gears, the second ring gears having rotary shafts, respectively, which are rotatably supported on the housing but securely mounted on the second arms, respectively, and which have axes of rotation in coaxial relationship to the axes of rotation of the first and second arms, respectively. The second ring gears are held in meshing engagement to the planetary gears, respectively, and are different in pitch from the first ring gears. Between the planetary gears a permanent magnet is provided to have a pair of polarities different in the axial direction thereof. A plurality of electromagnetic coils are provided equiangularly in each of the first ring gears and each has an axis extending radially so that the electromagnetic coils are able to generate different polarities at portions radially inwardly and outwardly of the first ring gear, the polarities being held in face-to-face relationship to the polarities, respectively, of the permanent magnet in the radial direction of the first ring gear. This results in the fact that the polarities of the electromagnetic coils are circumferentially successively changed if the electromagnetic coils are circumferentially successively energized. Therefore, there are generated circumferentially successively attractive forces which cause the planetary gears to rotate about their axes of rotation and to revolve about the center axis of the first ring gears. The second ring gears are thus caused to rotate about their axes of rotation by the reason that the first and second ring gears are held in meshing engagement to the planetary gears, respectively, and the second gears are different in pitch from the first ring gears. This means that first and second arms can be pivoted with respect to each other about their axes of rotation at reduced speeds correspoding to the gear ratio of the first and second ring gears.
Difficulties are however encountered in that the prior-art geared motor are unnecessarily made bulky and large in size since the geared motor requires a pair of first ring gears, a pair of second ring gears and a pair of planetary gears which are arranged in the axial direction of the geared motor as described above. For the reason that the polarities of the electromagnetic coils are circumferentially successively changed if the electromagnetic coils are circumferentially successively energized, all the teeth of the gears are magnetized and thus come to attract a large amount of iron powder on the surfaces thereof, thereby causing the surfaces of the gears to be worn and damaged.